As a matter of definition, an "optical image bar" comprises an array of optical picture element ("pixel") generators for converting a spatial pattern, which usually is represented by the information content of electrical input signals, into a corresponding optical intensity profile. Although there are a variety of applications for these image bars in a number of different fields, a significant portion of the effort and expense that have been devoted to their development has been directed toward their application to electrophotographic printing, where they may prove to be a relatively low cost and reliable alternative to the flying spot raster scanners which have dominated that field since its inception. Optical displays may also benefit from the use of such image bars, although their application to that field has not been a principle focus of the research that has been performed.
Several different types of optical image bars have been proposed, including some which embody electrically addressable LED arrays (see "Linear LED Array Has 300 Pixel/In. Resolution," Electronics Week, Jan. 21, 1985, p. 21), others which embody electro-mechanical spatial light modulators (see a commonly assigned U.S. Pat. No. 4,492,435 of M. E. Banton et al., which issued Jan. 8, 1985 on a "Multiple Array Full Width Electro Mechanical Modulator"), and still others which embody electrooptic spatial light modulators (see another commonly assigned U.S. Pat. No. 4,281,904 of R. A. Sprague et al., which issued Aug. 4, 1981 on a "TIR Electro-Optic Modulator with individually Addressable Electrodes"). Also see, "Light Gates Give Data Recorder Improved Hardcopy Resolution," Electronic Design, July 19, 1979, pp. 31-32; /"Polarizing Filters Plot Analog Waveforms," Machine Design, Vol. 51, No. 17, July 26, 1979, p. 62; and "Data Recorder Eliminates Problem of Linearity," Design News, Feb. 4, 1980, pp. 56-57. While these known image bars are based on diverse technologies, they all have finite spatial addressing capacities (i.e., they are "discrete image bars") because there are only certain, predetermined coordinates ("addresses") in image space upon which they can center pixels. Consequently, if the diameter of the individual output pixels produced by such an image bar is less than their pitch (i.e., the center-to-center displacement of the pixels on the output image plane), there inherently are interpixel intensity nulls. The spatial frequency of these intensity nulls sometimes is sufficiently high that they can be masked by overexposing the image, but that is not a particularly attractive null suppression technique because it degrades the image quality, requires additional optical power, and reduces the overall exposure latitude.
Some of the more interesting image bar proposals relate to TIR (total internal reflection) electrooptic spatial light modulators. In keeping with the teachings of a commonly assigned U.S. Pat. No. 4,396,252 of W. D. Turner, which issued Aug. 2, 1983 on "Proximity Coupled Electro-Optic Devices," such a modulator characteristically comprises a set of laterally separated, individually addressable electrodes which are maintained closely adjacent a reflective surface of an optically transparent electrooptic element, such as a lithium niobate crystal. A modulator of that type normally is operated with substantially the full width of its electrooptic element being illuminated by a linearly polarized, transversely collimated light beam, so when voltages representing the adjacent pixels of a linear pixel pattern (i.e., a line length set of data samples) are applied to its individually addressable electrodes, the wavefront of the light beam has its phase and polarization spatially modulated in accordance with the pixel pattern for a given line of an image. As a general rule, of course, successive sets of data samples are sequentially applied to the electrodes, thereby causing the modulator to serially modulate the wavefront of the light beam as a function of time in accordance with a time sequenced series of pixel patterns.
For image bar applications of such modulators, prior proposals typically have contemplated the use of Schlieren imaging optics for imaging the modulator onto its output image plane. The frequency plane filtering of a Schlieren imaging system effectively transforms the spatially modulated output radiation of the modulator into a series of correspondingly modulated intensity profiles, but there are embodiments in which a polarization analyzer may be used alone or in combination with a Schlieren stop to read out the pixel pattern represented by the data samples applied to an electrooptic modulator. Thus, as used herein, the phrase "electrooptic image bar" applies to all image bars which embody electrooptic spatial light modulators, regardless of whether the modulators are read out by spatial filtering and/or by polarization filtering.
There have been several significant developments which have reduced the cost and increased the reliability of TIR electrooptic spatial light modulators. Among the improvements that are of particular relevance to image bar applications of these modulators are a "differential encoding" technique that is described in a commonly assigned U.S. Pat. No. 4,450,459 of W. D. Turner et al., which issued May 22, 1984 on "Differential Encoding for Fringe Field Responsive Electro-Optic Line Printers," and an electrical interconnect strategy that is described in a commonly assigned U.S. Pat. No. 4,367,925 of R. A. Sprague et al., which issued Jan. 11, 1983 on "Integrated Electronics for Proximity Coupled Electro-Optic Devices." Briefly, it has been shown that the number of electrodes which such a modulator requires, when used in an image bar having a predetermined resolution, may be reduced by a factor of almost two if the input data samples are differentially encoded on a line-by-line basis prior to being applied to the modulator. Furthermore, it has been demonstrated that more or less conventional VLSI circuit technology can be employed to integrate the modulator electrodes with their addresssing and drive electronics, thereby facilitating the orderly and reliable distribution of data samples to the relatively large number of electrodes which customarily are required for reasonably high resolution printing.
Electrooptic image bars intrinsically are spatially coherent devices. Axially illuminated TIR electrooptic spatial light modulators (i.e., those wherein the incident radiation propagates in a direction that is essentially parallel to the optical axis of the modulator) are especially well suited for use in higher resolution image bars, but they inherently tend to produce interpixel intensity nulls because they spatially modulate the incident radiation by diffractively scattering optical energy into positive and negative diffraction orders which are more or less angularly symmetrical about a zero order or unmodulated component. These positive and negative diffraction orders (collectively referred to herein as "higher order diffraction components") define the upper and lower spatial frequency sidebands, respectively, of the modulated radiation, so they coherently contribute to the effective spatial modulation bandwidth of the modulator, provided that their relative phase is preserved. Unfortunately, however, whenever such spatially coherent radiation is brought to focus to form an image, adjacent pixels of opposite phase destructively interfere with one another, thereby producing undesireable interpixel intensity nulls. For example, differential encoding produces adjacent pixels of opposite phase.
Others who have attempted to develop essentially null-free image bars embodying axially illuminated TIR electrooptic spatial light modulators have recognized that the unwanted interpixel intensity nulls are caused by destructive interference, so their work is especially noteworthy. As described in a commonly assigned U.S. Pat. No. 4,437,106 of R. A. Sprague, which issued Mar. 13, 1984 on "Method and Means for Reducing Illumination Nulls in Electro-Optic Line Printers," one of the prior null suppression proposals suggests scattering light into the null regions in accordance with a pattern having an angular orientation and/or a spatial frequency which tends to inhibit the ability of the unaided eye to resolve the nulls, even when the imaging is performed at normal exposure levels. This approach preserves the internal spatial coherency of the output radiation of the image bar, while reducing the observable affects of the nulls. Another commonly assigned U.S. Pat. No. 4,483,596 of S. W. Marshall, which issued Nov. 20, 1984 on "Interface Suppression Apparatus and Means for a Linear Modulator," describes an alternative proposal pursuant to which a polarization retardation plate or the like is provided for orthogonally polarizing the positive and negative diffraction orders of the modulated output radiation of the image bar, thereby preventing them from destructively interfering with one another. That effectively suppresses the interpixel intensity nulls, but it does so at the cost of reducing the effective spatial bandwidth of the image bar by a factor of two because it destroys the relative phase information between the positive and negative diffraction orders. Moreover, it may be relatively difficult and expensive to take full advantage of this orthogonal polarization concept in practice because of the wide range of incident angles at which light from different points along an image bar of appreciable width would fall on the polarization retardation plate.